C ++ - & CRTP）。类型擦除与多态性 [英] C++ -& CRTP . Type erasure vs polymorphism

问题描述

好的，我们去。我尝试使用 CRTP 模板，以便从我的应用中删除多态性的需要。我使用类似下面的aproach

  template< RealType> class Base 
 {
 
 void doSomething（）
 {
 static_cast< RealType>（this） - > doSomethingImpl（）
} 
 
 class Derived1：public Base 
 {
 void doSomethingImpl（）
 {
 / * do something，for real * / 
} 
} 
 
 class Derived2：public Base 
 {
 void doSomethingImpl（）
 {
 / * do something else * / 
} 
} 
  

这个aproach，如果我理解正确，允许我的类没有vtable，是直接的，不需要额外的间接。

现在想象我要把所有的Derived＃类存储在容器中。让我们说一个向量。

第一种方法：我可以创建一个非模板SuperBase类，Base从该类继承并存储在容器中。

然而在我看来，如果我想这样做，我必须在SuperBase中做doSomething虚拟。我的目标主要是没有一个vtable。

第二种方法：我使用类型擦除，例如boost :: any来存储我的元素。
但是，我没有看到一个方法，我可以迭代的元素，并调用doSomething对他们，因为使用boost ::任何，我需要知道对象的实际类型，当迭代。 / p>

你认为我想做的是甚么可能？

似乎是因为doSomething（）是Base的一部分，但除了使用继承，我看不到一个方法来做。 p>

解决方案

我的目标主要是没有一个vtable。

如果你想要这个，除非你实现你自己的虚拟调度系统（这可能会低于编译器做的） ，你被困于模板，也就是编译时多态性。正如名称所示，为了使用它， 一切都必须在编译时知晓 。如果您需要根据运行时事件（例如用户输入）进行决策， 您想要运行时多态性 。

我不禁要问：为什么要避免vtables？如果你是这样决定的，为什么你不是在C编程？）

OK here we go. I'm trying to use the CRTP template in order to remove the need of polymorphism from my app. I use an aproach like the one bellow

template <RealType> class Base 
{

    void doSomething()
    {
         static_cast<RealType>(this)->doSomethingImpl()
    }

class Derived1 : public Base
{
    void doSomethingImpl()
    {
        /* do something, for real */
    }
}

class Derived2 : public Base
{
    void doSomethingImpl()
    {
        /* do something else */
    }
}

This aproach, if I understood correctly, allows my classes to have no vtable, so function calls are direct and don't require the extra indirection.

Now imagine I want to store all my Derived# classes in a container. Let's say a vector.

First approach : I can create a non-template SuperBase class from which Base inherits and store that in the container.

However it seems to me that if I want to do that, I will have to make doSomething virtual, in SuperBase. And my goal is mainly not to have a vtable.

Second approach : I use type erasure, i.e. something like boost::any to store my elements in the Vector. But then, I don't see a way I can iterate over the elements and call doSomething on them, because to "use" boost::any, I need to know the real type of the object when iterating.

Do you think what I want to do is even possible ?

Seems to me that it is because doSomething() is part of Base but except using inheritance, I can't see a way to do it ....

解决方案

And my goal is mainly not to have a vtable.

If you want this, then, unless you implement your own virtual dispatch system (which would likely be inferior to what the compiler does), you're stuck with templates, a.k.a. compile-time polymorphism. And as the name says, in order to use this, everything must be known at compile-time. If you need to make decisions based on runtime events (like, e.g., user input), you want runtime polymorphism.

I can't help but have to ask: Why do you want to avoid vtables? (And if you're so determined, why aren't you programming in C?)

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